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Software-defined radios point way for simpler direction finding

Advances in hardware and software are making it increasingly
practical for open-source analysts to carry out radio direction
finding. Writing for Jane's, Tony Roper examines the impact the new
tool is having on open-source analysis.

Key Points

The development of software to automate the use of time
difference of arrival (TDoA) analysis using networked
software-defined radios (SDR) is making direction finding (DF)
increasingly practical as a tool for open-source analysts.

DF can be used as another tool for corroboration or to identify
previously unknown transmitters, although at present the level of
accuracy possible can be limited and is far below the capabilities
of governments and militaries.

The future effectiveness of this technique is likely to be
dictated more by the number of SDRs that are available for use
around the world than by software or hardware limitations.

Identifying the location of a radio transmitter is a core
capability of military and government signals intelligence units.
However, open-source analysts have encountered challenges in
replicating this capability. The development of new software for an
existing network of online software-defined radios (SDRs)
incorporating global navigation satellite system (GNSS) timestamps
has rapidly changed this situation in 2018. It is increasingly
practical for an open-source analyst with an internet connection to
conduct rudimentary DF.

TDoA measures the time-of-flight differences of a signal
received at different locations. The basic principle is that the
difference in the arrival time of a signal at two receivers,
obtained by cross-correlating the two signals, can be used to
calculate a hyperbola of possible locations for the transmitter.
Conducting measurements from more than two receivers enables the
calculation of multiple hyperbolae, and the point at which these
intersect identifies the location of the transmitter.

Jane's conducted two case studies to assess the
effectiveness of DF using the KiwiSDR TDoA plug-in. The first used
a signal from a known transmitter site used by the Royal
Netherlands Navy at Goeree-Overflakkee island, southwest of
Rotterdam. The signals being transmitted were encrypted STANAG 4285
messages.

The capability afforded by networked SDRs will almost certainly
remain orders of magnitude less effective than the capabilities of
military and government signals units. With the software and
hardware highly likely to improve, the key constraint on the
technique's effectiveness in the future is likely to be the number
of SDR receivers available in different parts of the world.
Nonetheless, it represents a new tool for open-source analysts and
one that has yet to be fully exploited.